In speed sensorless vector control, an AC output voltage outputted from a power converter to the AC rotary machine has an extremely low voltage value in an extremely low speed region in which speed of an AC rotary machine is extremely low. Moreover, the speed sensorless vector control may induce a setting error in rotary machine constant of an AC rotary machine to be used, an error in voltage value of the AC output voltage caused by a short-circuit prevention period between upper and lower arms including semiconductor elements configuring the power converter, and an error in voltage value of the AC output voltage caused by on-voltage drop of each of the semiconductor elements configuring the power converter. Therefore, the voltage value error in AC output voltage becomes extremely influential in the extremely low speed region in which the AC output voltage has an extremely low voltage value. On the other hand, in the speed sensor less vector control, an angular-frequency estimate value being an estimate value of angular frequency of the AC output voltage is calculated assuming that the AC output voltage corresponds to a voltage instruction value, and therefore when the voltage value error increases due to the above factors, an error in angular-frequency estimate output of the AC output voltage also increases, leading to instability in control. Therefore, the speed sensorless vector control has a problem that the angular-frequency estimate output of the AC output voltage should be stably calculated even in the extremely low speed region.
To meet the problem, according to the following patent document 1, first estimate means is provided, which generates an angular-frequency estimate value of an AC output voltage outputted from a power converter to an AC rotary machine based on a q-axis current instruction value Iq* and a q-axis current detected value Iq, and second estimate means is provided, which generates an angular-frequency estimate value of an AC output voltage based on the q-axis current instruction value Iq* and a d-axis current instruction value Id*, and when the angular-frequency estimate value of the AC voltage given by the first estimate means is equal to or less than a predetermined value, the first estimate means is switched to the second estimate means, and then control is continued. The second estimate means of the patent document 1 obtains an acceleration instruction value by a product of the d-axis current instruction value Id* and the q-axis current instruction value Iq*, and integrates the acceleration instruction value with an angular-frequency estimate value just before switching as an initial value, thereby obtains the relevant angular-frequency estimate value. In the patent document 2, monitor means is provided, which compares an angular-frequency estimate value being output from an angular-frequency estimate section to a preset lower-level set value, and performs switching by using switch means such that when the angular-frequency estimate value becomes equal to or less than a lower-limit level set value, set angular frequency as an output signal from function generation means is used in place of the angular-frequency estimate value.
Patent document 1: JP 2001-238497A, particularly paragraph 0005 thereof
Patent document 2: JP 8-80098A, particularly abstract thereof